Flexography applies broadly to printing processes that utilize a flexible substrate bearing an elastomeric or rubbery relief surface. Flexographic printing plates are well known for use in printing, particularly on surfaces which are soft and easily deformable, such as packaging materials, e.g., cardboard, plastic films, etc. Flexographic printing plates can be prepared from photopolymerizable materials comprising an elastomeric binder or binders, one or more monomers and a photoinitiator. Photosensitive elements used to manufacture flexographic printing plates generally have at least one layer of photopolymerizable material interposed between a support and a peelable coversheet or a multilayer cover element. Upon imagewise exposure to actinic radiation, the at least one photopolymerizable layer polymerizes in the exposed areas causing insolubilization of the exposed photopolymerizable composition. Development, such as with a suitable solvent or by thermal blotting, removes the unexposed areas of the photopolymerizable layer leaving behind a printing relief which can be used for flexographic printing.
After development, the relief image printing plates are mounted to print cylinders to perform the printing operation. When the printing plate is mounted to the print cylinder, a high degree of accuracy of mounting of the plate to the cylinder is necessary so that the relief image is properly aligned, or registered. Accurate standards must be followed in affixing the printing plate to the cylinder to prevent images on the finished work from being blurred or overlapping. Registration is especially critical when more than one color of ink is involved. For example, when printing in full color, there are typically four separate printing plates, each having its own ink. Each printing plate has a different coverage area that contributes to the overall image being printed. If the printing plates are not in proper registration, the image will appear fuzzy.
Historically, in packaging printing, to mount the flexographic plates to the printing cylinder, vinyl sheets having adhesives coated on each side, commonly referred to as “stickyback,” have been used. Plates are mounted with a partial or an entire layer of stickyback between the plate and the printing cylinder. However, these vinyl sheets are incompressible, thin and tend to vary in caliper. In addition, the printing plate, printing cylinder, gears, substrate and impression cylinder each tend to have variation in tolerances in surface smoothness and height or thickness. Such inaccuracies dictate the use of increased pressure in the printing process, but the increased pressure can in turn cause deterioration in print quality due to yielding under pressure of the flexographic printing plates. Undesirable results including a dirty appearance of printing and inaccurate reproduction of halftones, (e.g., oval dots or halos around characters and images) may occur. The increased use of thinner plates formed by photopolymerization techniques, further accentuate the resulting problems associated with printing with non-uniform materials.
In an effort to overcome the shortcomings of the stickyback sheet, layers of synthetic polymeric foam as backing materials or as tapes have been suggested for use in mounting flexographic plates onto the printing cylinder. The polymeric foam materials are compressible and thus have sufficient cushioning effect to compensate for the variations in thickness or surface height of the plate, plate cylinder, gears, substrate and impression cylinder. In addition, the foam materials are selected to have sufficient resiliency to rebound rapidly and repeatedly to the original dimensions during printing. However, during use, these polymeric foam materials can fatigue, because the foam loses compressibility and resiliency, and cannot rebound to its original dimensions. In order to compensate for the loss of compressibility, other compressible materials, such as elastomeric materials, have also been used. Furthermore, it has also been suggested to use elastomeric materials that have various relief surfaces such as open-cells (i.e., open cells having total void volume in excess of 40 percent as described for example in U.S. Pat. No. 5,894,799 to Bart et al., the subject matter of which is herein incorporated by reference in its entirety) or longitudinal protrusions, as described for example in U.S. Pat. Nos. 6,247,403 and 6,666,138 to Randazzo, the subject matter of each of which is herein incorporated by reference in its entirety.
Thus it can be seen that it is important that the cushion element be sufficiently resilient to rebound rapidly and repeatedly from the compressed state to the original dimensions during printing with no or only minimal fatigue over time. The cushion element must also be sufficiently compressible to compensate for variations in thickness or surface height of the plate, plate cylinder, gears, substrate and impression cylinder during printing. However, there are still additional improvements needed to provide a cushion element that also reduces smutting and improves image registration, especially during printing by metal-backed flexo plates.
Therefore, it would be desirable to provide an improved cushion element that overcomes the deficiencies of the prior art. To that end, the inventors of the present invention have determined that a beneficial result can be obtained by integrating low friction surfaces directly into the cushion element itself for use on flexographic presses, especially those designed for metal-backed plates flexographic plates.
The improved process of the present invention does not require any spraying of the printing plate itself and does not expose the press operator to any volatile chemicals. In addition, the improved process of the present invention does not require any expensive coating to the back of the plate. Finally, the improved process of the present invention enables the use of thinner (and less expensive) plates for printing. Along with thinner plates, the present invention also increases the tonal range of the imaged plates.